This invention relates to a process for stabilizing particulate alkali metal peroxy salts selected from the group consisting of peroxycarbonates, peroxypyrophosphates, peroxytripolyphosphates and/or peroxymonosulfates, by coating the salts with a solid coating material. The invention also relates to resulting particulate alkali metal peroxy salts which are stabilized by the applied coating.
It is well known that peroxy compounds can be used as bleaching compounds in powdered detergent mixtures. Sodium perborate tetrahydrate or sodium perborate monohydrate are usually used as bleaching agents in common household detergents, since these materials are relatively resistant to decomposition in a pure detergent medium. However, other alkali metal peroxy salts, particularly peroxycarbonate (i.e. percarbonate or PCS), are also being used with increasing frequency. Although these alkali metal peroxy salts, and percarbonate in particular, are stable in the pure dry state, they have the disadvantage that they readily decompose in the presence of moisture with consequent loss of active oxygen.
It has been suggested in the prior art to overcome this disadvantage of the alkali metal peroxy salts by coating the peroxy compound with stabilizing materials or mineral protective films. The suggested solutions of the prior art are described in further detail hereinafter by means of the example of percarbonate, e.g. sodium percarbonate (PCS).
In U.S. Pat. No. 4,526,698 the use of a borate is suggested as coating agent for coating percarbonate, which borate may optionally additionally contain an alkali metal silicate. A similar coating material of different boric acids which, if necessary, may also contain alkali metal silicate is described in U.S. Pat. No. 4,321,301. Apart from these attempted solutions of using boron compounds and alkali metal silicates, coating materials of mineral salts have also been recommended according to the state of the art. U.S. Pat. No. 4,325,933, for example, describes the coating of sodium percarbonate with alkaline earth metal salts which are applied onto the sodium percarbonate particles from an aqueous solution. U.S. Pat. No. 4,105,827 and Published United Kingdom Patent Application No. GB 1,538,893, moreover, describe the stabilization of alkali metal peroxy salts by coating with a mineral protective film of sodium carbonate with other mineral salts, in particular sodium sulfate; according to Published United Kingdom Patent Application No. GB 1,538,893, this mineral coating layer can additionally contain sodium silicate.
Although the above attempted solutions according to the state of the art provide a substantial improvement in the stability of alkali metal peroxy salts such as sodium percarbonate, the problems, particularly as regards sodium percarbonate, are not solved in a desirable manner by the solutions suggested according to the state of the art. This is attributable in particular to the fact that percarbonate, for example, can never be completely dry since, apart from the residual water from the production process, water is always formed spontaneously in the PCS itself as a result of the decomposition of hydrogen peroxide. In the case of PCS, it should moreover also be taken into account that it is not a clearly defined homogenous compound but always represents a mixture of compounds, some of which contain water of hydration, with the following formulae:
Na.sub.2 CO.sub.3 . 1.5 H.sub.2 O PA2 Na.sub.2 CO.sub.3 . 1.5 H.sub.2 O . H.sub.2 O PA2 Na.sub.2 CO.sub.3 . 2 H.sub.2 O . H.sub.2 O PA2 Na.sub.2 CO.sub.3 . 2 H.sub.2 O PA2 Na.sub.2 CO.sub.3. X H.sub.2 O
The moisture which adversely affects the stability of PCS therefore not only originates from the base powder which introduces a substantial proportion of the moisture into the detergent composition via the individual components such as zeolite, linear alkyl sulfonates, soap, carboxymethyl cellulose, soda, optical brighteners etc. but a certain proportion of the harmful moisture thus originates from the percarbonate itself. An optimum coating layer for alkali metal peroxy salts, in particular for percarbonate must therefore be capable not only of keeping moisture from the percarbonate on the outside in the form of a dense protective film but it must simultaneously be capable of sufficiently firmly bonding the moisture resulting in the percarbonate itself from the water of crystallization and/or the decomposition. In addition, the coating material should not reduce the rate of dissolution of the alkali metal peroxy salt to any major extent as is frequently observed in the case of coating layers with a high alkali metal silicate content according to the state of the art.